1. Field of the Invention
The present invention relates to a semiconductor light emitting device and an illuminating device using it.
2. Description of the Related Art
In recent years, a nitride semiconductor including nitrogen in V group element has received attention in a field of a semiconductor light emitting device such as a light-emitting diode and a laser diode using p-n junction, and it is researched and developed in many places. The reason why the nitride semiconductor receives attention is that the nitride semiconductor, including AlN, GaN, and InN, is a direct transition semiconductor and, in a ternary mixed crystal and quaternary mixed crystal, it is possible to emit light from infrared up to deep-ultraviolet by changing a bandgap by setting the composition appropriately.
However, in manufacturing a semiconductor light emitting device using a nitride semiconductor, because it is difficult to manufacture a substrate for epitaxial growth which is made of nitride semiconductor and has high quality and large area, it is necessary to use, e.g., a sapphire substrate and a silicon carbide substrate as a substrate for epitaxial growth. However, in such a case, heteroepitaxial growth is imposed, and it is difficult to grow a nitride semiconductor thin-film having a flat surface. So, a density of threading dislocation in the nitride semiconductor thin-film is as much as 109 to 1011 cm−2. Because the threading dislocation causes descent of internal quantum efficiency of a semiconductor light emitting device, a technique for reducing dislocation and a selection of a material of an emission layer which is insusuceptible to dislocation are researched so as to increase internal quantum efficiency of the semiconductor light emitting device.
As to the technique of reduction of the dislocation, a GaN layer is centrally researched, and various techniques, such as the introduction of a low-temperature buffer layer, epitaxial lateral overgrowth using a selective growth mask, and an anti-surfactant method which controls a surface structure by adding anti-surfactant (e.g. Si), which comprised of an impurity atom for inducing three-dimensional growth, to a surface of a foundation layer (e.g. a GaN buffer layer) are examined, and a substrate for epitaxial growth having low lattice mismatch is examined. It is reported that it is possible to reduce the threading dislocation density in the GaN layer to about 105 cm−2. However, in a AlGaN ternary mixed crystal, it is known that an effect which can be obtained by using the above-mentioned technique of the reduction of the dislocation is small, so it is necessary to examine the technique of reduction of the dislocation more in AlGaN which is used as a material of the semiconductor light emitting device for emitting light in an ultraviolet region.
On the other hand, as an emission layer material which is insusuceptible to the threading dislocation, InGaN ternary mixed crystal has been in the limelight. InGaN is strong in immiscibility, and a composition of In in a crystal becomes unevenness with increase of the composition of In, and a carrier injected in an InGaN layer recombines in a region having high composition of In before it is captured by the threading dislocation. So, it becomes possible to increase internal quantum efficiency dramatically in spite of high degree of threading dislocation by using InGaN in the emission layer.
However, if In is included in the emission layer material, an emission wavelength shifts to a long wavelength side. So, many semiconductor light emitting devices which emit light of ultraviolet region adopt AlGaN which does not include In as a material of the emission layer, so the internal quantum efficiency is low.
Here, in order to increase the quality of an AlGaN layer, it is known that, when a single-crystal substrate for epitaxial growth is a sapphire substrate, it is desired that the AlGaN layer grows on an AlN layer as a foundation layer. This is because, by growing the AlGaN layer on the AlN layer a lattice mismatch between the AlGaN layer and the foundation layer becomes small as compared with a case where the AlGaN layer grows directly on the sapphire substrate, and the threading dislocation is less likely to occur in the AlGaN layer. Furthermore, this is because distortion is added to the AlGaN layer in a direction that the AlGaN layer is compressed, and adjacent threading dislocations in the AlGaN layer are united to each other, which enables reduction of the dislocation and prevents generation of crack. It is also known that, when the AlN layer is a foundation layer, it is preferable that the AlN layer grows directly on the sapphire substrate without a low-temperature buffer layer.
Also, in recent years, a semiconductor light emitting device which uses AlGaInN quaternary mixed crystal as a material of an emission layer to emit light of ultraviolet region has been in the limelight (see Japanese Patent Application Laid-Open No.9-64477). Although the AlGaInN layer includes In, it is reported that it is possible to set an emission peak wavelength to a wavelength region below 360 nm and it is possible to improve the internal quantum efficiency to the same level as the InGaN layer. Furthermore, the AlGaInN layer enables a crystal to grow in a temperature range more than 800 degrees C. at which the InGaN layer cannot allow a crystal to grow, so it has an advantage that it becomes easy to obtain a crystal with higher quality.
So, it is expected to realize an ultraviolet-emitting device which emits light at higher efficiency by combining the technology for increasing quality of the AlGaN layer and the technology which uses AlGaInN quaternary mixed crystal as a material of the emission layer.
However, an n-type GaN layer or an n-type AlGaN layer used for an n-type nitride semiconductor layer that is to be a foundation of an emission layer grows in a temperature range more than 1000 degrees while a growth temperature for forming an emission layer including In is usually 600 to 750 degrees C. So, in order to form the emission layer after forming the n-type nitride semiconductor layer, it is necessary to interrupt the growth to change substrate temperature. Therefore, a surface of the n-type nitride semiconductor layer may be polluted before the emission layer is formed. Furthermore, because the lattice mismatch between the n-type AlGaN layer which is an n-type nitride semiconductor and the AlGaInN layer which is an emission layer is large, the internal quantum efficiency may be reduced even if an n-type nitride semiconductor layer which is comprised of an n-type AlGaN layer having high quality is formed and a material having high efficiency as a material of the emission layer is used.